JP 2003-56416 A published by Japan Patent Office in 2003 discloses a conventional leak diagnosis device (or leak detecting device) for a fuel vapor treatment unit. This leak diagnosis device periodically integrates detected internal pressures of an evaporation system and makes a determination on a leak from the evaporation system on the basis of an integrated value. A leak diagnosis is carried out herein for a period in which a “positive pressure” is maintained when the evaporation system is sealed immediately after stoppage of an engine, or for a period shorter than that period.
By the same token, JP 2003-74422 A published by Japan Patent Office in 2003 discloses a conventional leak diagnosis device. This leak diagnosis device carries out a leak diagnosis by comparing an integrated value, which is obtained by periodically integrating internal pressures of a fuel vapor treatment unit, with a leak criterion value during a leak diagnosis period. It should be noted herein the leak diagnosis period means a period in which a “positive pressure” is maintained when an evaporation system is sealed immediately after stoppage of an engine, or for a period shorter than that period. Moreover, when the internal pressure of the system becomes equal to or higher than a predetermined pressure during the leak diagnosis period, the leak diagnosis device temporarily opens the system. Thus, a large amount of a vapor gas is prevented from being discharged to outside air when the fuel vapor treatment unit is opened to outside air after termination of the leak diagnosis.
The aforementioned conventional arts are based on the premise that the pressure in the evaporation system rises and becomes positive when the evaporation system is sealed immediately after the engine has been stopped. This is because an exhaust system is at a high temperature and a large amount of the vapor gas is generated immediately after the engine has been stopped. However, when the engine is operated for a long period, highly evaporable light components in fuel have already evaporated, and the generation amount of the vapor gas after stoppage of the engine is small. In this case, the pressure in the evaporation system may become negative as the evaporation system is cooled after the engine has been stopped. Thus, since the pressure in the evaporation system is not always positive, the aforementioned conventional arts sometimes make it difficult to carry out the leak diagnosis appropriately.